Behavior

Is fear contagious?

Recent data suggest that olfactory signals may be involved in interactions between horses and people, facilitating emotional communication.

Olfaction is the sense most widely used by animals to communicate. Although this sense has been studied mainly in the context of reproduction within a species, ethologists from the National Research Institute for Agriculture, Food and Environment (INRAE) and IFCE wanted to see if there was a relationship between odor and human–horse interactions.

The researchers collected fear- and joy-related odors from 30 volunteers. To do so, they placed cotton pads under the armpits of participants while they watched 20-minute video clips depicting situations of fear or joy. To evoke fear, an excerpt from the horror film Sinister was shown. To elicit joy, the participants saw a montage of various excerpts from comedy sketches, musicals and comedies.

The researchers then presented 43 horses with a pad impregnated with the human odor of fear, the human odor of joy, or no odor at all (control). They measured behavior, heart rate and cortisol levels in saliva during various tests:

• 2 interaction tests: 1 in which the experimenter groomed the horse, and an approach test in which the experimenter stood in a defined area to measure whether the horse would approach
• 2 ‘fear’ tests: a suddenness test in which an umbrella was suddenly opened next to the horse, and a test in which an unknown object was placed near the horse

Horses exposed to the smell of human fear exhibited higher levels of fearful behavior and were less inclined to interact with humans. They showed stronger reactions to the sudden opening of an umbrella and reported more gazes at the unfamiliar object. Their heart rates were also higher.

In the interaction tests, they had less physical contact with humans, both in the approach test and during grooming.

These findings suggest that horses adopted a fear-like emotional state when exposed to that human smell. The results point to emotional contagion of fear through chemical cues between 2 different species, humans and horses. While this phenomenon has already been demonstrated in dogs, the researchers believe this is the first time such emotional communication has been documented in another animal species.

These findings highlight the importance of olfactory signals in interactions between different species and provide new insights into the effect of domestication on emotional communication between humans and animals.

“There is a chemical communication of emotions, and it crosses the species barrier,” said Léa Lansade, INRAE research director and a co-author of the study. “When exposed to the odors of humans experiencing fear, horses in turn exhibit fear responses through a mechanism of emotional contagion.” MeV

FOR MORE INFORMATION:
Jardat P, Destrez A, Damon F, et al. Human emotional odours influence horses’ behaviour and physiology PLoS One 2026;21(1):e0337948.